1. Field of the Invention
The present disclosure relates generally to an image forming apparatus and a process cartridge having a cleaning unit.
2. Description of the Background Art
Typically, an image forming apparatus employing electrophotography includes a photoconductor for forming a toner image thereon, and a cleaning unit for removing toner particles remaining on the photoconductor after transferring the toner image to a transfer member such as a sheet. Such cleaning unit may employ a blade cleaning method, which uses a rubber blade to remove toner particles from a photoconductor by contacting the blade to the photoconductor.
However, such blade cleaning method may have some drawbacks. For example, if a blade and a surface of photoconductor do not contact precisely, toner particles may pass through a tiny space between the blade and the photoconductor. Such drawback may be suppressed by contacting the blade to the photoconductor with a higher pressure. However, if the blade is pressed to the photoconductor with a higher pressure, the blade may curl, which may result in insufficient cleaning of the photoconductor, appearing as streaks. Further, if the blade is pressed to the photoconductor with a higher pressure, the blade more likely scrapes the surface of photoconductor and thereby reducing a lifetime of the photoconductor, which is undesirable.
Further, in view of recent market demand for higher quality image, toner particles having a smaller diameter have been introduced to the market. Further, in view of a constant need to reduce toner manufacturing costs and enhance toner transfer performance, more and more image forming apparatuses employ spherical toners having toner particles of more or less uniform spherical shape produced by polymerization instead of pulverized toner having toner particles of non-uniform shape. However, the blade cleaning method described above may be less effective in cleaning spherical toner particles having a smaller diameter compared to pulverized toner particles.
One background art technique discloses an electrostatic brush cleaning method for coping with such drawbacks of the blade cleaning method. The electrostatic brush cleaning method may have a good level of cleaning performance for spherical toner having a smaller diameter, and may reduce mechanical abrasion of a photoconductor (i.e., scraping of surface coat of a photoconductor) caused by friction with a blade.
Such electrostatic brush cleaning method may use a cleaning brush and a recovery roller, in which the cleaning brush having brush fibers contacts a surface of photoconductor to remove toner particles remaining on the photoconductor, and the recovery roller contacts the cleaning brush to remove toner particles adhering to the cleaning brush.
When conducting a cleaning operation by an electrostatic brush cleaning method, a given voltage is applied to either or both the cleaning brush and the recovery roller. For example, the cleaning brush may be supplied with a voltage having one polarity, which is opposite to a polarity of charged toner particles on the photoconductor, by which toner particles on the photoconductor can be removed and transferred to the brush fibers of the cleaning brush by electrostatic force. Such mechanism can enhance cleaning performance for spherical toner particles having a smaller diameter.
Generally, toner image or toner particles developed on a photoconductor has one polarity (referred as a first polarity), and a transfer unit of an image forming apparatus is supplied with a voltage having another polarity (referred as a second polarity) opposite to the first polarity of developed toner image or particles. When the transfer unit applies the second polarity to the developed toner image having the first polarity, a toner image is transferred from the photoconductor to a transfer member (e.g., transfer sheet). In such image transfer process, an electric charge having the second polarity is applied to toner particles developed on the photoconductor and having the first polarity.
Such toner particles developed on the photoconductor may have a different potential value, that is, some toner particles may have a smaller potential value (referred as weakly charged toner particles). If the electric charge having the second polarity is applied to such weakly charged toner particles, such weakly charged toner particles may change their polarity from the first polarity to the second polarity. Consequently, after a toner-image transfer process, toner particles remaining on the photoconductor may be a mixture of two types of toners, that is, toner particles having the first polarity (i.e., polarity of developed toner image) and toner particles having the second polarity.
As noted above, a cleaning brush for cleaning toner particles remaining on a photoconductor is supplied with a voltage having a given polarity. Accordingly, some toner particles remaining on the photoconductor have a polarity opposite to a voltage polarity applied to the cleaning brush, and some toner particles on the photoconductor have a polarity which is the same as voltage polarity applied to the cleaning brush. Therefore, toner particles having a polarity the same as a voltage polarity applied to the cleaning brush may not be attracted to the cleaning brush, and thereby the cleaning brush cannot remove such toner particles, resulting in poor cleaning performance.
A cleaning brush has been devised to cope with such drawbacks. For example, a cleaning brush is supplied with a voltage having one given polarity from a power source, and the same cleaning brush has brush edges frictionally electrified to another polarity opposite to the applied given polarity, by using a friction of brush fibers of the cleaning brush and a photoconductor. Such configured cleaning brush may attract toner particles having a polarity opposite to the voltage polarity applied to the cleaning brush, and also attract toner particles having a polarity the same as the voltage polarity applied to the cleaning brush. Such mechanism may suppress phenomenon that toner particles on the photoconductor are not be captured by the cleaning brush
After recovering and attracting toner particles to the cleaning brush as above described, toner particles adhering to such cleaning brush may be removed and transferred to a recovery roller, which is supplied with a given voltage from a power source. Specifically, the recovery roller is supplied with a first recovery voltage set higher than a voltage applied to the cleaning brush with a same polarity, by which toner particles having a polarity opposite to a voltage polarity applied to the cleaning brush and adhered on the cleaning brush may be electrostatically attracted to the recovery roller from the cleaning brush.
Then, assuming that toner particles adhered on the cleaning brush still have a given level of electric charges, the recovery roller is supplied with a second recovery voltage having a polarity opposite to the first recovery voltage by using a switching device, which is used to switch voltage polarity applied to the recovery roller. Accordingly, toner particles adhering to the cleaning brush and having a polarity the same as voltage polarity applied to the cleaning brush can be electrostatically attracted to the recovery roller from the cleaning brush. With such process, the recovery roller may recover toner particles having positive and negative polarity adhering to the cleaning brush.
However, if the recovery roller is made of a conductive metal material such as stainless steel (SUS) or the like, some toner particles may not be effectively recovered by the recovery roller from the cleaning brush, and such residual toner particles remaining on the cleaning brush may re-adhere to the photoconductor, degrading cleaning performance.
Further, the cleaning brush, contacting the recovery roller made of SUS, may have a potential substantially similar to a potential of the recovery roller. In such a case, the recovery roller and the cleaning brush may have a smaller potential difference, by which toner particles may not be effectively attracted to and recovered by the recovery roller.
Further, when the cleaning brush is observed after recovering toner particles having a polarity the same as voltage polarity applied to the cleaning brush by the recovery roller, some toner particles still adhere to the cleaning brush. Such toner particles may continue to adhere to the cleaning brush until a subsequent printing job. If such next printing job is not conducted for a long period of time, an electric charge amount of such adhered toner particles may become substantially zero. If the electric charge amount of such adhered toner particles becomes zero, such toner particles having zero potential may not be electrostatically attracted to the recovery roller but may remain on the cleaning brush even if a next printing job is conducted.
Based on further observation of such cleaning brush and recovery roller, it is found that some toner particles on the cleaning brush are strongly charged when toner particles are recovered from the cleaning brush to the recovery roller, in which the recovery roller may introduce electric charges to toner particles on the cleaning brush and then toner particles adhering to the cleaning brush are strongly charged with a polarity the same as the voltage polarity applied to the cleaning brush. Accordingly, such strongly charged toner particles having a polarity the same as the voltage polarity applied to the cleaning brush may re-adhere to the photoconductor.
Further, it is also observed that some toner particles invert their polarity with an introduction of electric charges by the recovery roller when toner particles are recovered to the recovery roller from the cleaning brush. Accordingly, such polarity-inverted toner particles may not be recovered by the recovery roller, and may continue to adhere to the cleaning brush after one printing job has been completed.
In view of such phenomenon, a recovery roller having a metal core made of a conductive material such as SUS and a surface layer made of insulating material was prepared as a high-resistance recovery roller, and such high-resistance recovery roller shows some effects as follows.
For example, such high-resistance roller may have an effect of suppressing electric charge introduction to toner particles when the toner particles are recovered by a high-resistance recovery roller from a cleaning brush, and also have an effect of suppressing polarity inversion of toner particles adhered on the cleaning brush. With such configuration, toner particles may not remain on the cleaning brush or may not re-adhere to the photoconductor.
At the same time, however, such high-resistance recovery roller may not effectively recover toner particles under a lower temperature/lower humidity environment, especially when toner particles are input (or used) in greater amount.
In view of such phenomenon, a surface potential of such high-resistance recovery roller under lower temperature/lower humidity environment after recovering toner particles from the cleaning brush was measured, and such measured surface potential of the high-resistance recovery roller was found to be lower than an initial surface potential of the high-resistance recovery roller by about several hundred volts, wherein the initial surface potential was measured before conducting a toner recovery operation.
If the surface potential of the high-resistance recovery roller declines, a potential difference between a cleaning brush edge of a cleaning brush and a surface of the high-resistance recovery roller decreases. As a result, the high-resistance recovery roller may not effectively attract toner particles adhering to the cleaning brush, and therefore toner particles may not be effectively recovered by the high-resistance recovery roller.
Although the cause of such surface potential decline of a high-resistance recovery roller is not yet known, such surface potential decline of high-resistance recovery roller was not observed when a cleaning brush was operated without inputting or adhering toner particles to a cleaning brush. Accordingly, it may be said that such surface potential decline may be related to toner recovery.
For example, such surface potential decline may occur as follows: When toner particles adhere to the high-resistance recovery roller, toner particles may give electric charges having a polarity opposite to a voltage polarity applied to the high-resistance recovery roller to a surface of the high-resistance recovery roller, by which a surface potential of the high-resistance recovery roller may decrease. Further, toner particles adhering to the high-resistance recovery roller and having a polarity opposite to voltage polarity applied to the high-resistance recovery roller may be scraped by a recovery blade, which contacts the surface of the high-resistance recovery roller. During such scraping process by the recovery blade, electric discharge may occur on the high-resistance recovery roller, decreasing a surface potential of the high-resistance recovery roller.
Consequently, toner particles may not be effectively recovered when a high-resistance recovery roller is used in certain lower temperature/lower humidity environments, which is undesirable.